(noun) A (generally homo)sexual act in which one person penetrates their partner's anus as that person defecates, causing a power struggle between the two solids. Can be either intentional or unintentional. Takes its name from Madison, WI, the sjw capitol of the world.
Mark: Dude, last night I was super horny, so naturally I called up my fuckbuddy. He said he was down, but hadn't "gone" in a while. Long story short, I was too horny to care and ended up with a Madison Mudslide. That's like the third time this month!
Gabe: "He" is a trans-phobic micro-aggression. Please use the non-gender specific "they."